Examples of droplet ejection devices include ink jet printers. Ink jet printers typically include an ink path from an ink supply to a nozzle path in a printhead module. The nozzle path terminates in a nozzle opening in a surface of the printhead module from which ink drops are ejected. Ink drop ejection is controlled by pressurizing ink in the ink path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electro statically deflected element. A typical printhead module has an array of ink paths with corresponding nozzle openings and associated actuators, and drop ejection from each nozzle opening can be independently controlled. In a drop-on-demand printhead module, each actuator is fired to selectively eject a drop at a specific pixel location of an image as the printhead module and a printing substrate are moved relative to one another. In high performance printhead modules, the nozzle openings typically have a diameter of 50 micron or less, e.g., around 25 microns, are separated at a pitch corresponding to 100-600 nozzles/inch or more, have a resolution of 100 to 600 dpi or more, and provide drop sizes of about 1 to 70 picoliters (pl) or less. Drop ejection frequency is typically 10 kHz or more.
Hoisington et al. U.S. Pat. No. 5,265,315, the entire contents of which is hereby incorporated by reference, describes a printhead module that has a semiconductor printhead module body and a piezoelectric actuator. The printhead module body is made of silicon, which is etched to define ink chambers. Nozzle openings are defined by a separate nozzle plate, which is attached to the silicon body. The piezoelectric actuator has a layer of piezoelectric material, which changes geometry, or bends, in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path.
Printing accuracy is influenced by a number of factors, including the size and velocity uniformity of drops ejected by the nozzles in the head, as well as the alignment of the head relative to the printing substrate. In printers utilizing multiple printhead modules, head alignment accuracy is critical to printing accuracy as errors in alignment between printhead modules or between printhead modules and other components of a droplet ejection device can result in erroneous droplet placement relative to droplets from different printhead modules in addition to erroneous drop placement relative to the substrate.
In many applications, particularly in droplet deposition devices utilizing multiple printhead modules, printhead modules are aligned by iteratively adjusting a printhead module's position and checking nozzle location either by direct optical inspection of the printhead module or by printing and examining a test image. This procedure is repeated whenever a printhead module is removed or replaced.